The following disclosure relates generally to the field of medical imaging systems and, more specifically, to the application of deformable image registration techniques in a computer software tool for creating a synthetic computed tomography (CT) image.
Tomography refers to a technique for capturing a two-dimensional slice or cross-sectional image of an object, through the use of radiation or any kind of penetrating wave. The word tomography is likely derived from the Greek words, tomos (slice, section) and grapho or graphein (to write). Computed Tomography (CT) refers to a medical imaging technique that uses X-rays and computer processors to collect and display a series of two-dimensional images or tomograms of an object. Additional computer processing and mathematics can be used to generate a three-dimensional image. Magnetic Resonance (MR) imaging refers to a tomographic technique that uses a powerful magnetic field and a radio frequency transmitter to capture detailed images of organs and structures inside the body.
Positron Emission Tomography (PET) refers to an imaging technique that produces a three-dimensional image of the body. To conduct a PET scan, a short-lived radioactive tracer isotope is injected into the patient. As the isotope in the tracer decays, it emits a positron. When a positron interacts with electrons in the body, the interaction produces a pair of gamma photons. When the photons pass through the body and eventually reach a set of scintillator crystals in the scanning device, the resulting bursts of light are detected, creating an image.
Attenuation refers to the gradual loss of energy as a signal passes through a medium. In the context of Computed Tomography, attenuation of the X-ray energy occurs as the X-rays pass through different tissues and structures in the body. Similarly, for Positron Emission Tomography, attenuation of the photon energy occurs as the gamma photons pass through different tissues and structures in the body. Different tissues have different densities and, thus, produce different amounts of attenuation. The attenuation coefficient describes the extent to which a particular material or tissue causes a loss of energy.
Combination or hybrid imaging techniques are particularly useful, especially when the two images can be aligned or registered. For example, the anatomical images obtained by CT can be combined with the metabolic information shown in a PET image, to reveal both the structure of the body and its biochemical activity on a single display. The integration of PET and CT scanners in a single gantry has allowed for the simultaneous capture of both types of images. A combined PET-MR scanner has the potential to significantly change healthcare and revolutionize clinical practice because it allows the simultaneous visualization of both molecular and anatomical imaging. MR provides better soft-tissue contrast differentiation than CT. Combined PET-MR scanners can correlate temporally a PET measurement (including metabolic processes, blood flow, binding potential, distribution volume, etc.) with an MR measurement (blood flow, diffusion and perfusion, etc.) and thus create new possibilities for more specific disease assessment. Hybrid PET-MR systems will likely become the advanced imaging modality of choice for neurological studies, for the analysis of certain forms of cancer and stroke, and for the emerging study of stem cell therapy.
Image registration is the process of aligning different sets of data into a common coordinate system. The different data sets may include raw image data from different times, different viewpoints, or different types of scanning devices. Image registration allows the user to more accurately compare data from different sets. For example, the images from CT scans taken at different times, for example, can be aligned in order to identify changes occurring over time. Image registration can also be performed on data sets obtained from two different types of scans or modalities. For example, a CT image may be aligned or co-registered with an MR image, in order to display the information gathered by both scans. In mathematical terms, image registration is the process of determining the spatial transform function that maps the points in a first image to homologous points in a second image.
Segmentation is the process of identifying and classifying data found in a digitally sampled representation, such as the data acquired from a medical imaging device. Image segmentation is typically used to locate discrete objects and boundaries in the image. For example, segmentation helps identify anatomical features, locate tumors, and measure tissue volumes. The Insight Tool Kit (ITK) is an open-source software tool for performing image registration and segmentation. The ITK includes a framework of separate components, functions and routines that can be selected separately and customized by the user to assist with particular types of image registration and segmentation.
Hybrid PET-MR systems are currently in use for imaging the head and brain. Attenuation correction maps are not directly available for processing combined PET-MR data into accurate images. One of the most significant problems associated with PET imaging is photon attenuation caused by the patient's own tissues, which reduces the apparent radiopharmaceutical activity, causing image degradation, and potentially preventing the patient from receiving appropriate diagnosis or therapy. One existing protocol includes the extra step of obtaining a separate CT scan for reference, in order to use the CT dataset to obtain an attenuation correction map that can be used in the PET image reconstruction algorithms. Obtaining a reference CT scan, however, exposes the patient to an additional dose of radiation, increases health care costs, takes more time, and may introduce errors to the extent the patient changes posture or position during the additional scan procedure. There is a need, therefore, for improved systems and methods of building attenuation correction maps for hybrid imaging systems.